For the measuring of volumetric flow, magneto-inductive flow measuring devices make use of the principle of electrodynamic induction: Charge carriers moving perpendicular to a magnetic field in the medium induce a voltage in electrodes which likewise are essentially perpendicular to the flow direction of the medium, and perpendicular to the direction of the magnetic field. The measurement voltage produced in the electrodes is proportional to the flow volume of the medium averaged over the cross section of the measuring tube, and therefore proportional to the volume flow rate. If the density of the medium is known, the mass flow in the pipeline and/or the measuring tube can be determined. The measurement voltage is usually tapped via an electrode pair, which is situated in that area of the measuring tube in which the maximum magnetic field strength—and thus the maximum measurement voltage—is to be expected. The electrodes are typically galvanically coupled with the medium; however, magneto-inductive flow measuring devices with capacitively coupled contactless electrodes are also known.
The measuring tube can either be produced from electrically conductive material, e.g. stainless steel, or from an electrically insulating material. If the measuring tube is made of an electrically conductive material, the portion of it coming in contact with the medium must be coated with an electrically insulating liner. Depending on temperature and medium, the liner can be made, for example, of a thermoplastic, a thermosetting plastic or an elastomeric plastic. There are, however, also magneto-inductive flow measuring devices with a ceramic lining.
An electrode can be roughly divided into an electrode head, which is at least partially in contact with a medium which flows through the measuring tube; and an electrode shaft, which is almost completely inserted in the wall of the measuring tube.
The electrodes are, along with the magnet system, the central components of a magneto-inductive flow measuring device. With regard to the design and arrangement of the electrodes, it is important to ensure that they can be installed in the measuring tube as easily as possible, and that, subsequently, no leakage problems occur during the measurement process. The electrodes should furthermore exhibit both a sensitive and low-disturbance registering of measurement signals.
In addition to those electrodes that serve to pick up a measurement signal, further electrodes are often built into the sides of the measuring tube, e.g. a reference or grounding electrode for the electrical reference potential between the measuring device and the medium, or a medium-monitoring electrode for detecting partially-filled or empty measuring tubes.
Ultimately, electrode construction types can be divided into two categories:                those inserted and installed in the measuring tube externally and        those inserted and installed in the measuring tube internally.        
With regard to the first type, the design of the electrode head is limited by the diameter of the bore: an external mount is only possible when the diameter of the electrode head is smaller than or equal to that of the bore in the wall of the measuring tube in which the electrode is placed. Preferable among externally mounted electrodes are rod electrodes, for example as described in European Patent EP 0 892 252 A1. Rod electrodes are preferable for installation in measuring tubes with small nominal diameters; that is to say, in areas where internal mounting is difficult or impossible to achieve on account of the small dimensions. The use of rod electrodes is not, however, solely limited to magneto-inductive flow measuring devices with small nominal diameters.
For the second electrode construction type, the restrictions concerning the dimensions of the electrode head do not apply; in this case the electrode shaft must simply be designed in such a way that it can be mounted in the bore. Depending on its application, the electrode head can vary widely in both size and shape. Internally mountable electrodes typically have, for example, mushroom, lentil or plate-shaped electrode heads.
Electrodes are normally set liquid-tight in a bore in the wall of a measuring tube. This is achieved through rows of frustoconically shaped sealing lips along the length of the electrode in the area of the electrode shaft, as is, for example, described in the prior noted European Patent, EP 0 892 252 A1. Rod electrodes can be mounted without difficulty in a plastic or plastic-lined measuring tube with a large nominal diameter. The rows of frustoconically shaped sealing lips along the length of the electrode serve both to hold the electrode in place and to seal the interior of the measuring tube off from the external environment.
When a rod electrode is pressed into a pre-made bore in the wall of a measuring tube, mechanical stresses are produced, especially if the diameter of the bore is smaller than the outside diameter of the rod electrode. With a plastic measuring tube wall, this has the effect that the plastic in stress-free areas “flows” and the electrode anchors shape-interlockingly in the bore. The electrode is thereby secured against being pushed out of the measuring tube as a result of an operating pressure. The measuring tube is furthermore sealed off from the outside environment by the shape-interlocking envelopment of the electrode shaft.
In the case of low wall thickness, for example in the case of measuring tubes with small nominal diameters, e.g. DN 15, standard electrodes with a reduced number of sealing lips are used. Trials have showed that in such cases, the prevention of an electrode being pushed out by higher operating pressures in the measuring tube can no longer be guaranteed. Furthermore, through the electrode's being pressed in, the bore will be widened by the sealing lip nearest the electrode head, so that sufficient material is no longer available for the shape-interlocking anchoring of the electrode and the following sealing lips are no longer wedged against the wall of the bore and no longer close off the wall of the bore with their maximum diameter. Especially in the case of plastic walls, there is a tendency for a part of the plastic to give way to the interior of the measuring tube due to mechanical stresses and form bulges there, which locally decreases the diameter of the measuring tube and in turn influences the flow of the liquid.